refactor(storage): apply code review suggestions

Author: angelcaamal

storage/removeBucketIamMember.js

@@ -66,7 +66,7 @@ function main(
         if (role.members.length === 0) {
           policy.bindings.splice(index, 1);
         } else {
-          policy.bindings.index = role;
+          policy.bindings[index] = role;
         }
 
         // Updates the bucket's IAM policy

storage/system-test/iam.test.js

@@ -31,8 +31,7 @@ const roleName = 'roles/storage.objectViewer';
 // Condition
 const title = 'match-prefix';
 const description = 'Applies to objects matching a prefix';
-const expression =
-  'resource.name.startsWith("projects/_/buckets/bucket-name/objects/prefix-a-")';
+const expression = `resource.name.startsWith("projects/_/buckets/${bucketName}/objects/prefix-a-")`;
 
 before(async () => {
   await bucket.create();